The catalytic dehydrogenation of alkanes (paraffin hydrocarbons) to produce alkenes (olefin hydrocarbons) is an important and well known hydrocarbon conversion process in the petroleum refining industry. This is because alkenes are generally useful as intermediates in the production of other more valuable hydrocarbon conversion products. For example, propylene can be used in the production of polymers and propylene glycol, butylenes can be used in the production of high octane motor fuel and isobutylenes can be used to produce methyl-t-butyl ether, a gasoline additive.
The catalytic dehydrogenation of alkanes is an endothermic reaction. The reaction is very fast and reversible and conversion rates are limited by the thermodynamic equilibrium conditions. High temperatures and low pressures favorably displace the reaction toward the formation of alkenes.
Numerous patents describe state of the art systems for the catalytic dehydrogenation of alkanes. For example, U.S. Pat. No. 4,381,417 describes a catalytic dehydrogenation system in which a radial flow reactor is employed and U.S. Pat. No. 5,436,383 describes a catalytic dehydrogenation system in which either a fixed bed, moving bed, or fluid bed reactor can be employed. Because of the fast and endothermic nature of the catalytic alkane dehydrogenation reaction, prior art processes all require multiple reactors or reactor stages to achieve a sufficient yield of alkene product. Additionally, conventional catalytic dehydrogenation systems require multiple heaters to supply the heat of reaction. Typically a preheater and multiple reactor interheaters are used. The interheaters are positioned between the reactors to ensure that at the entrance of each of the reactors, the temperature conditions necessary for the endothermic dehydrogenation reaction are met. In an alternative prior art process, a set of catalytic dehydrogenation reactors are operated in a cyclic non-steady-state mode with regeneration of a catalyst bed every 10 to 30 minutes, as described in U.S. Pat. No. 6,392,113. The catalyst bed is heated during regeneration and this heat is used to carry out the dehydrogenation reaction. Reactors are large and multiple reactors in parallel are needed for large plant sizes. Frequent cycling of the system can lead to operational and maintenance problems and the non-continuous system is less thermally efficient than a continuous process.
It has long been recognized in industry that conventional catalytic dehydrogenation systems suffer from a variety of drawbacks. For example, in conventional catalytic dehydrogenation systems relatively large reactors are necessary to achieve equilibrium conversion. This increases the complexity and capital costs of the catalytic dehydrogenation system. Capital costs are also increased in conventional systems by the need to have multiple reactors. A further drawback is that the high temperatures that are required to shift the equilibria favorably to alkene products also promote rapid deactivation of the catalyst by coking. The high temperatures can also lead to thermal cracking of the alkanes—i.e. undesirable non-selective side reactions resulting in formation of byproducts with a broad range of carbon numbers, which complicates separation of the product stream. The formation of heavy byproducts can foul the reactors, with the result that the catalytic dehydrogenation system has to be shut-down periodically and cleaned. In order to limit the amount of fouling, the heavy byproducts are separated from the unconverted alkane prior to recycle of the unconverted alkane using a front-end distillation column.